# 1 Jan 2001 was Monday
week_day = 1  # 1 based indexing, so 1 -> Monday, 2 -> Tuesday, ..., 7 -> Sunday
month_days = 31  # January has 31 days
month = 1  # 1 -> January, 2 -> February, ..., 12 -> December
year = 2001

req_number_of_sundays = 0

while year != 2101:
    if (month % 2 == 1 and month <= 7) or (month % 2 == 0 and month > 7):
        month_days = 31
    else:
        if month == 2:
            if year % 4 == 0:
                month_days = 29
            else:
                month_days = 28
        else:
            month_days = 30

    for day in range(1, month_days + 1):
        if week_day > 7:
            week_day = 1
        if day == 1 and week_day == 7:
            req_number_of_sundays += 1
        week_day += 1

    month += 1
    if month > 12:
        month = 1
        year += 1

print("The number of Sundays are: " + str(req_number_of_sundays))

# Output: 
#
# The number of Sundays are: 172